Actual Barlow Magnifications

A while back I did some preliminary tests using the moon and concluded that my barlows were indicating a power greater than marked. To evaluate barlow magnification, I affixed a steel tape to a board and placed it at a distance of about 400 feet. Using 11 EPs and two barlows (1.5X & 2.0X) I measured the FOV of each combination. Included in the group were a Stratus 8mm and Hyperion 13mm which could be used either as 1.25" or 2.0" EPs. When used as 2" EP the Stratus gave 1.55 and 2.09 for the two barlows and the Hyperion gave 1.43 and 2.05 respectively. However, with the 2.0/1.25" adaptor I got an average of 1.7X and 2.32X respectively. These values were also the approximate average I got for all EPs, 10 of which were 1.25". The adapter raises the EPs approximately 3/8" above the edge of the barlow, thus increasing the effective power of the barlow. However, the 2" EP, a 42 mm GSO Superwide yielded valaues of 1.75 and 2.39. Lesson learned is that it may not be the barlow that causes the apparent increase in magnification, rather the amount your adapter raises the EP above the flush point of the Barlow. More to come after some additional testing.

Good testing Jud, thanks for sharing the results.
Which Barlows were used in the test? Name brand that is.

Judson:

Indeed, the effective magnification of a barlow lens depends upon the distance from eyepiece to the barlow. There are some basic equations which I do not have handy which can be used to compute the actual magnification of the barlow.

A basic rule of thumb says that if you place a 2x barlow in front of the diagonal on a refractor or SCT it will provide about 3x though again this is very rough and depends on the barlow...

Jon